signals into the cell. GPCRs mediate many important signaling functions in the central nervous system (CMS) and mediate the effects of many therapeutic and abused drugs. A basic mechanism by which these receptors are regulated is by ligand-induced endocytosis. This process reduces the number of activated receptors at the cell surface therefore regulating receptor signaling. While endocytosis is strongly regulated by physiological ligands and drugs, it is generally believed that recycling is a nonregulated process. New evidence from our laboratory and others are challenging this 'default1 hypothesis of GPCR recycling. We have developed a technique to investigate single insertion events at the cell surface by combining live total reflection microscopy (TIRF) with a pH sensitive extracelullar tag fused to the extracelullar terminus of GPCRs. Our preliminary results indicates that receptor recycling is a tightly regulated process involved in the homeostatic response to extracelullar stimuli. Furtheremore, regulation of recycling is dependent on receptor activation, which ultimately decreases the frequency of recycling events reducing receptor number at the cell surface. I propose to extend and further develop this novel approach to examine: a) The characteristics of the vesicles mediating receptor recycling, b) Identify the machinery involved in the fusion process, and its possible regulation by receptor activity, c) Investigate if the regulation of receptor recycling is a general process for GPCRs, d) Define and investigate the functionality of newly inserted receptors and the possible existence of a molecular complex associated to reinserted receptors. This work will be perfomed using an experimentally advantageous heterologous cell model, and the results will be latter examined in hippocamapal neurons. By investigating the regulation of receptor insertion to the cell surface, our work has the potential to provide fundamental new insight to key cellular events likely to be relevant to a wide variety of physiological and pathological processes. Obtaining the K99/ROO award will be an extraordinary opportunity to transition from my postdoctoral studies to an independent position in a university/research institution.